A computer network generally includes a number of devices, including switches, routers and hubs, connected so as to allow communication among the devices. The devices within a network are often categorized into two classes: end stations such as workstations, desktop PCs, printers, servers, hosts, fax machines, and devices that primarily supply or consume information; and network devices such as gateways, switches and routers that primarily forward information between the other devices.
Network devices ordinarily operate on a continuous basis. Each device has one or more circuit boards, a microprocessor and a memory, and runs a control program. In general, networks often include several different types of data switching and routing devices. These network devices may have different physical characteristics. New devices, with characteristics that are presently unknown, are constantly being developed. In addition, the characteristics of many network devices may change over time. For example, characteristics of the network devices change when subsystems like boards, network interface modules, and other parts are added or removed from a device.
Many networks are managed, supervised and maintained by a network administrator or network manager. To properly maintain a network, the network administrator needs to have up-to-date information available about the devices in the network and how the devices are interconnected. The Open Systems Interconnection (“OSI”) network reference model is useful in classifying network management information. Layer 2 and Layer 3 refer to the data link layer and the network layer respectively in the OSI model. Layer 2 connectivity information may be used by the network administrator to understand the physical topology of the network. The topology is a mapping that indicates the type of devices that are currently included in the network and how the interfaces of these devices are physically linked.
An effective network management system would show both the logical connections of network devices, and the physical connection information. Not only would the network management system be capable of logically identifying to which network a particular device is linked, based on Layer 3 information, but also indicate how individual devices are physically linked together, based on Layer 2 information. Thus, an administrator could determine the actual interface links or physical connectivity of each of the devices. This information may be critical in determining how the network can be expanded or upgraded, or in troubleshooting.
Certain proprietary discovery protocols have been developed to help identify how the interfaces of each device is physically linked within a network, based on Layer 2 information. For example, the Cisco Discovery Protocol (“CDP”), which is used in products that are commercially available from Cisco Systems, Inc., San Jose, Calif., is a low-level communication protocol that can help identify how devices are linked in a network. When a device supporting CDP sends a message to another device, the message includes a packet that contains its IP address and an interface descriptor. The interface descriptor represents the logical name of the port from which the message was sent. The information is recorded at the receiving device and may be used to determine how two devices are physically connected.
However, knowledge of the physical topology of the network may not be sufficient for diagnosing an error in the network. One technique of gathering information on errors and configuration in a network that uses Internet Protocol (“IP”) is path tracing for an IP packet. It is desirable for IP path tracing to be performed both at Layer 3 and Layer 2. Tools such as the UNIX-based traceroute computer program and the Windows-based tracert computer program are used to perform IP path tracing at Layer 3. However, techniques for direct IP tracing at Layer 2 that are suitable for a wide range of technologies including Ethernet, Local Area Network Emulation, Token Ring, and EtherChannel are not available. A Layer 2 IP path is hereafter referred to as a “data link path”. A data link path is important for network monitoring and diagnostics because the information obtained from IP path tracing performed at Layer 3 may be insufficient. For example, IP path tracing at Layer 3 identifies a path from router to router and does not identify the actual network devices such as LANs switches and bridges that an IP packet may traverse to get from a source device to a destination device.
Based on the foregoing, there is a clear need for a mechanism that can identify the data link path from a source device to a destination device on the network.
There is a specific need for a way to carry out IP Path tracing at Layer 2.